The present invention relates to a fixing device and an image forming apparatus.
In an electrophotographic system image forming apparatus, a fixing device that fixes a toner by heating a recording paper while pressurizing the paper against a fixing roller (fixing rotor) is employed. In a conventional fixing device, the fixing roller is heated throughout the entire length thereof. When fixing a recording paper of a narrow width, the recording paper is not brought in contact with ends of the fixing roller, and heat is not consumed. Therefore, it has sometimes been the case where the temperature of the ends of the fixing roller has abnormally risen when recording papers of a narrow width have been continuously fixed.
Particularly, in a fixing unit that causes induction heating by applying an alternating magnetic field to the fixing roller by means of an excitation coil, it is general to shorten the warmup time by reducing the thermal capacity of the fixing roller for a reduction in energy consumption. The recording papers to be fixed include large-size and small-size ones. In this kind of fixing unit, a temperature rise of a portion where the recording paper does not consume heat becomes significant when small-size recording papers of an identical width are continuously fixed, and this has sometimes caused a problem that a rise in the iron core temperature of the excitation coil is caused, resulting in making the heat generation in a portion through which the recording paper passes become unstable and incurring the deterioration of the excitation coil.
Accordingly, as shown in FIG. 9, U.S. Pat. No. 7,005,619 describes an invention that includes a demagnetizing coil 32 placed overlapping an excitation coil 31 through which the recording paper does not pass when a recording paper of a narrow width is fixed by a fixing roller. In the fixing device, a current that cancels a change in the magnetic flux caused by the excitation coil 31 flows through the demagnetizing coil 32 when the loop of the demagnetizing coil 32 is closed, by which the alternating magnetic field at an end where the recording paper does not pass is diminished, allowing the heat value of the fixing roller to be partially reduced.
FIG. 10 shows the distribution of the heat value in the axial direction of the fixing roller of FIG. 9. It can be understood that, although a virtually uniform generation of heat is observed since there is no demagnetizing effect in a state in which the demagnetizing coil is open as shown in the figure, the generation of heat in the portion where the demagnetizing coil is located largely drops when the demagnetizing coil is closed.
However, it is difficult to make the region through which the small-size recording paper passes coincide with an end of the demagnetizing coil in the conventional induction heating type fixing device. For example, if a distance between the end of the region through which the small-size recording paper passes and the end of the demagnetizing coil is excessively apart, there is a defect of the occurrence of the deterioration of the fixing roller due to the excessive temperature rise thereof as a consequence of a high generation of heat of the fixing roller and a temperature rise in the region through which the small-size recording paper passes. Conversely, if the distance between the end of the region through which the small-size recording paper passes and the end of the demagnetizing coil is excessively overlapped, there is a defect of the occurrence of defective fixing, irregular luster and offset as a consequence of a low generation of heat of the fixing roller and a fall in the temperature of the region through which the small-size recording paper passes. That is, a trouble due to overheating or the defective fixing due to a shortage in the generation of heat is to occur unless the transitional portion of the heat value coincides with the end of the recording paper.
If the fixing device is designed so that the end of the demagnetizing coil coincides with the region through which the end of the paper passes in order to avoid the inconvenience described above, a shift in the position of the paper to be conveyed tends to occur every apparatus, and the trouble described above has still occurred. Furthermore, it is possible to provide demagnetizing coils dividedly in multiplicity for the recording papers of which the paper widths largely differ as in the case of, for example, A4-size paper and A3-size paper. However, the demagnetizing coil cannot be divided for the recording papers of which the paper widths slightly differ as in the case of, for example, A4-size paper and B5-size paper or letter-size paper, meaning that proper management has not been achieved in actuality.